1. Field
The present invention relates generally to photovoltaic energy conversion systems, and more specifically to control of sub-arrays of photovoltaic energy conversion systems.
2. Background
Referring to FIG. 1, shown is a typical prior art photovoltaic (PV) system that includes a photovoltaic array that includes several sub-arrays (also referred to herein as branches). As shown, each of the sub-arrays may include a string of several PV panels (also referred to as modules), and each sub-array is typically coupled to a combiner via a corresponding fuse (as shown in FIG. 1) or a corresponding breaker. As depicted, the sub-arrays may be arranged in parallel and each sub-array may be stacked up to one of a variety of voltages (e.g., 1000 VDC) so that the DC output of the combiner provides an input to an inverter, which produces an AC voltage from the DC output of the combiner.
As depicted, when many sub-arrays are combined, there is the possibility that a damaged branch will be “back-fed” with current from the other healthy branches to which it is connected. This fault current, for a large number of combined branches, may far exceed the ampacity of the conductors used in the ailing branch. For this reason, conventionally applied protective combining (using protective devices such as fuses or breakers) is used to protect against excessive backfeed currents.
But there are several problems with this approach. For example, fuses (with associated disconnection equipment) and breakers are very expensive, and neither fuses nor breakers have “directional” settings (enabling them to open with a lower level of backfeed current), so they must be sized according to the maximum “forward” current of the sub-array. This means that it takes a substantial level of backfeed current to actuate these devices. This is problematic because backfeed current, even at small levels, can cause PV array damage. Two of the situations that can result in backfeed currents that are less than the forward current limit but high enough to cause PV array damage are a) sub-array shorts in low sunlight conditions and b) sub-array open-circuit mismatches.
In addition to the high cost and often ineffective protection that is provided by fuses or breakers, these typical protective devices only provide protection; thus the cost associated with these protective devices does not provide any added value (other than protection) to the operator of the system. As a consequence, there is a need in the art for alternative approaches to the system depicted in FIG. 1.